A critical application for integrated optic devices is the passive portion of repeaters for long haul fiber optic communications cable such as undersea cable contemplated for spanning the Pacific. Due to the extreme length of such cable, a large number of repeaters must be periodically located along the length of the cable to amplify and monitor signals traveling in both directions and to permit precise location of any fault in the cable.
It is contemplated that the passive portion of such a repeater will be a six terminal fiber optic device having two input terminals for receiving eastbound and westbound signals, respectively; two terminals for receiving optical pump signals; and two output terminals. The passive device must add to the eastbound signal a low level loop back signal from the westbound signal for monitoring and fault location. Similarly, it must add a loop back signal from east to west. The passive device must also introduce optical pump signals into both the eastbound and westbound fibers in order to facilitate the amplification of the signals these fibers carry. Because of the great length of the cable and the large number of repeaters which the length entails, the repeaters must perform these functions with low levels of back reflection and high levels of isolation.
Of the several functions performed by the passive repeater, one of the most demanding is tapping stable, low level loop back signals. The conventional approach to providing such signals is to pass two waveguides in close adjacency for a length dependent upon the desired degree of coupling. Energy from one core extends across the cladding to excite a diminished corresponding signal in the other core. This approach, however, does not work well when the desired degree of coupling is small. The coupled power decays exponentially with distance away from the core, rendering the core separation distance critical. For example a minus 45 dB tap typically has a core center-to-center separation of about 15 micrometers. However at such separations, the tail of the exponential field decay is both sensitive to processing of the device and dependent upon the polarization state of the signal. Accordingly, there is a need for a stable, low level optical tap which is reproducible and essentially independent of polarization.